Mar 7 2018
Getting one step closer to developing more energy-efficient memory devices, researchers at EPFL have used laser pulses to produce controllable stable skyrmions. The study has been reported in the Physical Review Letters journal.
Skyrmions forming on a surface
Credit: EPFL
A skyrmion is a collection of electron spins that resemble a vortex in specific magnetic materials. Skyrmions have the ability to occur individually or in patterns, called as lattices. Skyrmions get their name from Tony Skyrme, a British physicist who first proposed the presence of their elementary-particle counterparts in 1962, and have gained attention for their capacity to be applied in so-called “spintronic” devices, which exploit the spin as against the charge of electrons, thereby becoming far more miniaturized and energy-effective.
Considerable focus has been on the application of skyrmions in memory-storage technologies. Skyrmions can be quite stable and need lesser energy for writing on or erasing them—certain research works have demonstrated that producing and destroying skyrmions can be nearly 10,000 times more energy-effective when compared to traditional data-storage devices. Yet, it would mandate a quick and dependable process of controlling and regulating individual skyrmions.
At present, stable skyrmions have been successfully written and erased using laser pulses in the labs of Fabrizio Carbone and Henrik M. Rønnow at EPFL. The researchers used an iron-germanium alloy, which can offer skyrmions at about 0 °C, which is very close to the ambient temperature. This is, in itself, significant because a large number of these basic experiments normally occur at temperatures very low to ever be commercially relevant.
The scientists made the most of the super-cooling effect that occurs at the end of an ultrafast temperature jump, which is induced by itself in the alloy through an ultrashort laser pulse. At the time of the super-cooling, the skyrmions can be restricted to places in which they normally do not exist under traditional equilibrium conditions.
The researchers imaged the produced skyrmions through time-resolved cryogenic Lorentz electron microscopy, which has the potential to “see” magnetic domain structures and magnetization reversal mechanisms in real time and real space. This method is an advancement of static cryo-electron microscopy, for which the Nobel Prize in Chemistry was won by Jacques Dubochet in 2017.
What we did was apply a laser pulse to the alloy while it was kept at a temperature and external magnetic field that normally forbids the appearance of skyrmions. Individual skyrmions were seen to appear near the edges of the sample at every light flash. Furthermore, once the skyrmions were established, by adjusting the parameters in proximity to the transition between having the skyrmions and not having them anymore, laser pulses can be used to erase them via local heating-induced demagnetization.
Fabrizio Carbone
The scientists could write and erase skyrmions on the alloy in less than a few hundred nanoseconds to a few microseconds. Yet, the outcomes also indicate ways to engineer the super-cooling rates for more rapid control of the skyrmions, to a few picoseconds.
The energy barriers for manipulating skyrmions can be very small. This means that, if this was a memory-storage device, the energy consumption estimated by our experiments, in which the light properties were not yet tailored to optimize this parameter, is in the region of femto-Joules (quadrillionths of a Joule) per bit, already comparable to the most energy-efficient prototypes available.
Fabrizio Carbone
Although this research was a proof-of-concept work, the scientists could not abstain from imagining about the terms of applications. “We actually calculated the energy it requires, without any optimization in our experiment,” stated Carbone. “And we found that it was already is at the level of the least energy-consuming data-storage device to-date. If implemented into devices, this would mean something like your laptop’s battery lasting for about a month before needing to charge.”
University of Glasgow, Osaka Prefecture University, JST PRESTO (Japan), Hiroshima University, and EPFL Interdisciplinary Centre for Electron Microscopy (CIME) are the various contributors to the study.
The study was funded by NCCR MUST and Swiss National Science Foundation (Ambizione and Advanced Postdoc Mobility grants).
References
G. Berruto, I. Madan, Y. Murooka, E. Pomarico, G. M. Vanacore, J. Rajeswari, R. Lamb, P. Huang, Y. Togawa, T. LaGrange, D. McGrouther, H. M. Ronnow, F. Carbone. Speed limit of skyrmions optical creation/annihilation in FeGe. Physical Review Letters 02 March 2018.